Added application session services, along with initial restore and tracker implementations.

Harmonia.WinUI/Session/JsonSessionService.cs

@@ -0,0 +1,47 @@
+using System;
+using System.IO;
+using System.Text.Json;
+using System.Threading.Tasks;
+
+namespace Harmonia.WinUI.Session;
+
+public sealed class JsonSessionService : ISessionService
+{
+    private static readonly JsonSerializerOptions Options = new()
+    {
+        WriteIndented = true
+    };
+
+    private readonly string _filePath = Path.Combine(
+        Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
+        "Harmonia",
+        "session.json"
+    );
+
+    public ApplicationSession Session { get; private set; } = new();
+
+    public async Task LoadAsync()
+    {
+        try
+        {
+            if (File.Exists(_filePath))
+            {
+                await using var stream = File.OpenRead(_filePath);
+                Session = await JsonSerializer.DeserializeAsync<ApplicationSession>(stream) ?? new();
+            }
+        }
+        catch
+        {
+            Session = new();
+        }
+    }
+
+    public async Task SaveAsync()
+    {
+        string directoryName = Path.GetDirectoryName(_filePath)!;
+        Directory.CreateDirectory(directoryName);
+
+        await using var stream = File.Create(_filePath);
+        await JsonSerializer.SerializeAsync(stream, Session, Options);
+    }
+}